This invention relates to a photographic constituent for a flexographic printing plate.
Photosensitive constituents for a flexograpic printing plate generally comprise a support such as a polyester film having provided thereon a photosensitive elastomer composition containing a thermoplastic elastomer, at least one ethylenically unsaturated compound and at least one radiation-responsive initiator. In addition, a thin film, often called a slip layer or a protective layer, is provided, if necessary, on the photosensitive layer for the purpose of smoothing contact with a negative film. In making a flexographic printing plate from such a photosensitive constituent for a flexographic printing plate, it is customary to conduct, first, UV radiation exposure all over the constituent through the support (back exposure) to thereby form a thin uniform cured layer, then to conduct imagewise exposure (relief exposure) of the photosensitive resin layer through a negative film and wash away unexposed areas with a developing solvent to thereby obtain a desired image, i.e., a relief image which is used as a printing plate.
In a printing process by which fine photographs or the like are printed, it is required to form extremely fine halftone dots or fine lines as a relief image, but, in such cases, it is not easy to prepare an adapted printing plate. For example, when relief exposure is conducted in a large exposure amount in order to form a fine relief image, it makes a reverse image (depressions) having a small depth which, upon printing, sometimes suffers the disadvantage of the spreading of ink and fails to enable reproduction of a distinct print.
In addition, there often occurs the failure that reproduction of halftone dots or fine lines becomes larger than the dimension of those in the negative film, which exerts detrimental influences on printing reproducibility.
On the other hand, when the amount of relief exposure is decreased so as to avoid such failure, photocuring of the relief image becomes insufficient, and hence chips are easily formed in some cases by pressure of a washing brush or the like in a developing step, and the intended image-forming properties are not obtained.
In addition, while most of dried printing plates after development have residual tackiness on the surface thereof, such tackiness remaining on the printing plate causes adhesion between the printing plates to each other and, when delaminating the plates from each other, the relief image in some cases is damaged or extraneous matter cannot be easily removed, which exerts detrimental influences on printing quality. In such cases, tackiness on the surface of the printing plates can be removed by employing the so-called post-exposure treatment of irradiating the dried printing plate with UV radiation of shorter wavelength as described in JP-B-2-46935 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d).
In this process, however, there is involved a complicated procedure of discreetly determining the amount of the UV radiation of shorter wavelength. Because, if the irradiation amount of the UV radiation is insufficient, the degree of surface tack is insufficiently decreased, whereas, if too much, there results such deteriorated physical properties of the surface of printing plate that a number of cracks are formed during the carrying of the plate or in the course of printing, with the print quality being diminished.
The obtained printing plate is fixed around a plate cylinder using a pressure sensitive double coated tape or by drawing a vacuum so as to be subjected to a printing step. However, there must not be delamination between the support and the cured resin due to shear stress between the plate cylinder and the impression cylinder during printing. In addition, while the printing plate is removed after completion of printing for storage and preparation for re-printing, it is necessary for the printing plate not to be broken and to firmly adhere to the support even when a large force is applied to the cured resin layer upon being removed by holding part of the cured resin layer.
For this purpose, good adhesion is necessary between the support and the photosensitive resin layer and, as a general guide, an adhesion force of 1 kg/cm or more is desirable when forcibly delaminated at an angle of 180 degrees. From this point of view, it has become customary to provide an adhesive layer between the support and the photosensitive resin layer to thereby strengthen adhesion between the support and the photocured resin layer.
In general, a polyester film constituting the support and the photosensitive layer mainly containing a rubber component differ greatly from each other in polarity, and hence it is extremely difficult to select an adhesive showing affinity for both.
In addition, even when an adhesive is selected which shows at least some affinity for each of them, the adhesive layer is in direct contact with the photosensitive layer, and hence components in the photosensitive resin layer migrate with time into the adhesive layer and, as a result, the properties of the adhesive are often inhibited.
When an initiator or a monomer in the photosensitive resin layer migrates into the adhesive layer, the problem exists that the monomer causes polymerization in the adhesive layer as well in the exposing step, leaving curing contraction stress which may cause delamination at the interface between the support and the adhesive layer.
Accordingly, the adhesion force between the support and the photosensitive resin layer depends upon a combination of the compositions of the two and, in order to obtain a strong adhesion force, sufficient consideration must be given to this.
However, it has been difficult to find a combination of a photosensitive resin layer, an adhesive and a support, where the photosensitive resin layer exhibits excellent mechanical properties and relief-reproducing properties and yet retains strong adhesion to a support.
An object of the present invention is to provide a photosensitive constituent for a flexographic printing plate which shows good capabilities of forming extremely fine halftone dots and providing good printing quality in process printing, which enables the tackiness of the printing plate to be easily removed through post-exposure treatment, which avoids formation of cracks in the printing plate surface, in which adhesion force between the photosensitive resin layer and its support of a polyester film is strong enough not to cause any trouble under actual printing conditions, and which is stable with time.
As a result of extensive investigations, the inventors have found that the above objects can be achieved by using the following novel photosensitive elastomer composition in the photoconductive constituent for a flexographic printing plate, thus resulting in the present invention.
That is, the present invention provides a photosensitive constituent for a flexographic printing plate comprising a polyester film as support (B) and a photosensitive elastomer composition (C) and having an adhesive layer (A) therebetween,
wherein the photosensitive elastomer composition (C) comprises, as essential components, (a) 50% by weight or more of at least one thermoplastic elastomer comprising a mono-vinyl-substituted aromatic hydrocarbon and a conjugated diene, (b) 5 to 40% by weight of at least one diene-based liquid rubber, (c) 1 to 20% by weight of at least one ethylenically unsaturated compound having 2 unsaturated bonds within its molecule, and (d) 0.1 to 3% by weight of at least one photopolymerization initiator, and
wherein (1) the thermoplastic elastomer has an average ratio of vinyl bond units in conjugated diene segments thereof of 10 to 20 mol % and (2) the liquid rubber has a number average molecular weight (Mn) of 500 to 5,000 and an average ratio of vinyl bond units of 50 to 70 mol %.